1. Field of the Invention
This invention generally relates to an electrostatic capacitor type sensing device and more particularly to an electrostatic capacitor type sensing device used for the detection of water level in pools or rivers, the detection of a substance at a tip portion of a hand of a robot, the detection of substances on a belt conveyer in a factory or the like, and so on.
2. Description of the Prior Art
Various types of sensors or detectors have hitherto been developed for the detection of water level, or the detection of substances at a tip portion of a hand of a robot. One of such sensors utilizes the variation in electrostatic capacitance thereof. This sensor detects the existence or an approaching state of an object to be detected by using the variation in distributed capacitance, that is, stray capacitance thereof at the time the object approaches a capacitance portion thereof, with the result that the resonance frequency of a resonance circuit including the capacitance thereof changes.
In such measurement, since it is necessary to set the resonance frequency at a high value, for example, several kilohertz (kHz) to several megahertz (MHz), to increase the accuracy of the detection of an object, the value of the electrostatic capacitance of the sensor, which is a component determining the resonance frequency, should be reduced to an extremely small one. Further, in order to ensure high Q, the electrostatic capacitance of the sensor should be reduced. Usually, the electrostatic capacitance of such a sensor ranges from 0.1 pF to 5 pF. Thus, in a conventional electrostatic capacitance type sensor, a detection electrode 10 facing an object to be detected, a ground electrode 12 facing the detection electrode and an additional electrode 14 facing the ground electrode 12 are arranged in such a manner that there is provided a predetermined space between the electrodes 10 and 12 and another predetermined space between the electrodes 12 and 14, respectively, as shown in FIG. 9. Especially, each of the ground electrode 12 and the additional electrode 14 has a structure that a hollow cylindrical member is arranged in the direction of a common axis thereof such that the side surface of each of the electrodes 10, 12 and 14 does not face that of an adjacent one of the electrodes 10, 12 and 14. If the hollow inside of the ground electrode 12 and that of the additional electrode 14 are filled with synthetic resin or the like, the resonance frequency and the Q of the resonance circuit drop due to the increase in the electrostatic capacitance of the sensor. Thus, when these electrodes 12 and 14 are fixed in a casing of the sensor, the electrodes 12 and 14 are left hollow.
The electrostatic capacitance of the conventional sensors having such structure is small and thus the change of the electrostatic capacitance due to the variation in ambient temperature is small. However, when the sensitivity of a measuring circuit is increased to an extreme extent, even a slight change in the electrostatic capacitance affects the resonance frequency. Thus, in case of such a conventional sensor, it is difficult to sufficiently increase the detection sensitivity thereof. Such ill influence of the change in the electrostatic capacitance due to that in the temperature can be reduced to some extent by adding a capacitor having an inverse temperature characteristic thereto. However, when taking only such a measure, the influence of the change in the electrostatic capacitance is not yet sufficiently reduced. Thus, there have been no effective measures to prevent the lowering of the detection sensitivity due to the change in the temperature.
In order to resolve such a problem, Applicant of the instant application developed a three-electrode sensor provided two capacitors connected with each other in series (that is, a detection capacitor to be used for the detection of phase difference and a comparison capacitor to be used for comparison) which is arranged to supply a.c. signals to these capacitors and measures the change in electrostatic capacitance of the sensor as the change in phase. This three-electrode sensor is disclosed in Japanese Patent Application No. S61-201129 and in the above described related application in detail.
In this three-electrode sensor for phase comparison, the detection capacitor and the comparison capacitor are arranged to have substantially the same structure. Further, this sensor has an arrangement that ambient temperature affects both of the capacitors in a similar manner. Moreover, in this sensor, a symmetrical circuit is formed by connecting both of the capacitors in series and grounding the connection point between the capacitors. Thus, it is considered that this circuit operates in response to the change in ambient temperature such that the balance of stray capacitances of the capacitors is constantly maintained. However, in case of using the apparatus and method for supplying the detection capacitor and the comparison capacitor connected to each other in series and measuring the difference in phase between the capacitors, temperature compensation can be achieved to some extent but errors caused in measurement due to the change in temperature cannot be considerably reduced. For example, when temperature changes from 5.degree. C. to 50.degree. C., the error in measurement is more than 30%. Further, this sensor has a drawback that it is not suited for mass production because there is the necessity of selecting capacitors to be used in order to reduce the difference in performance between the capacitors used in this sensor as well as that of adjusting the positions of the capacitors and so on, for the purpose of improving the precision of measurement.
It is therefore an object of the present invention to provide an electrostatic capacitor type sensing device which can stably effect measurement with extremely little error due to the change in temperature without adjustment of composing elements thereof.